The present invention relates to an apparatus for indicating how dirty an air filter is that is disposed in an air stream for removing dirt therefrom.
Apparatus for monitoring the dirt-loading of an air filter in vacuum-cleaning apparatus such as vacuum cleaners, or of apparatus for cleaning the air of a room, are known. Such apparatus have a measuring arrangement for detecting the pressure differential ahead of and after the air filter of the air stream that is flowing through the filter. As the loading of the filter increases, the pressure differential increases. The detected pressure values are, however, subject to fluctuations, for example with vacuum-cleaning apparatus, since the volumetric flow of the air stream is greatly influenced by the use of the vacuum tool. If during a cleaning procedure the vacuum tool is disposed completely upon the surface that is to be cleaned, the suction air stream is significantly restricted, and changes greatly during the course of work depending upon whether the vacuum tool is partially or completely lifted from the surface that is to be cleaned. The measuring arrangement detects the pressure fluctuation and activates the indicator for showing that it is necessary to change the filter, although in fact it is not necessary for the filter to be replaced. Thus, the operator can only with great uncertainty estimate the filter loading and hence the point in time at which the filter should be cleaned or replaced. The indication for how dirty the air filter is is too imprecise.
Also with air-cleaning apparatus for cleaning the air of a room and/or for humidifying this air, the actual conditions are not advantageous. With regard to the generation of noise, and in order to avoid the formation of drafts, with such apparatus for cleaning the air of a room the velocity of the flow of the air stream, in other words the air volume that flows through the air-cleaning apparatus, is kept low. As a result of the air-conveying system of these apparatus, the vacuum generated by the blower is relatively slight, so that the pressure differential that occurs as air flows through the loaded filter is similarly very low and is therefore very complicated to determine. A further difficulty with air-cleaning apparatus that in addition humidify air is that water and lime deposits form in the filter.
It is an object of the present invention to improve an apparatus of the aforementioned general type in such a way that fluctuations in the volume of the air stream do not lead to incorrect indications of how dirty the air filter is.